Debugging Mono binaries with LLDB

Edit page on GitHub

Summary

It is possible to use LLDB to debug (with source code stepping, etc) a prebuilt Mono package (for example from Jenkins) without compiling it locally. This is useful if you want to be certain that what you’re debugging is exactly what a customer is running.

Steps

Let’s say we’re interesed in Mono 6.0.0.x (2019-02) (see: Mono source versioning)

  1. Download and install the latest green *.pkg file from the Jenkins per-commit builds for 2019-02
  2. Check out the Mono source code somewhere in your home directory, for example /Users/work/2019-02/mono
  3. Start debugging lldb mono example.exe

  4. Lookup a mono symbol to find out where the source code was when Jenkins built the package

    (lldb) image lookup -vn mini_init
1 match found in /Library/Frameworks/Mono.framework/Versions/Current/Commands/mono:
     ...
 CompileUnit: id = {0xffffffff00084fd3}, file = "/Users/builder/jenkins/workspace/build-package-osx-mono/2019-02/external/bockbuild/builds/mono-x64/mono/mini/mini-runtime.c", language = "c99"
    ...

  5. Set up a “source map” telling LLDB where the code really is

    (lldb) settings set -- target.source-map "/Users/builder/jenkins/workspace/build-package-osx-mono/2019-02/external/bockbuild/builds/mono-x64" "/Users/work/2019-02/mono"

  6. Set a breakpoint and start running

     (lldb) b mini_init
 (lldb) r
 * thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
  frame #0: 0x0000000100011bb8 mono`mini_init(filename="foo.exe", runtime_version=0x0000000000000000) at mini-runtime.c:4134:2 [opt]
    4131    MonoDomain *
    4132    mini_init (const char *filename, const char *runtime_version)
    4133    {
 -> 4134        ERROR_DECL (error);
    4135        MonoDomain *domain;
    4136        MonoRuntimeCallbacks callbacks;
    4137        MonoThreadInfoRuntimeCallbacks ticallbacks;
 Target 0: (mono) stopped.

Debugging more than one piece of code

To add symbols for some other shared library, use settings append -- target.source-map <old-path> <new-path> to add a second mapping, instead of settings set.